1. Field of the Invention
The present invention relates to the construction of a power semiconductor module used for a 3-level or 3-phase inverter or a resonance type inverter.
2. Description of the Related Art
FIG. 8 shows a circuit example of a 3-phase inverter for converting DC to AC, which uses a related art. The construction of this circuit is disclosed in JP-A-2002-247862. In FIG. 8, reference numerals 1 and 2 represent DC power sources connected to each other in series, and a terminal with a positive potential is represented by P, a terminal with a negative potential is represented by N, and a terminal with a neutral (middle) point potential is represented by M. In general, this DC power source may be an AC-powered DC source system, constructed by using diode rectifiers, large-capacity electrolytic capacitors, etc. (not shown).
Series-connected circuits of IGBTs (insulated gate bipolar transistors), corresponding to three phases, are connected between the positive terminal P and the negative terminal N. Diodes are connected in a reverse parallel manner (that is, in a back-to-back connection manner) to the IGBTs. In more detail, a series connection circuit 60 for a U-phase includes an upper arm comprising an IGBT 11 to which a diode 12 is connected in a reverse parallel style (that is, the cathode is connected to the collector of IGBT 11 and the anode is connected to the emitter) and a lower arm comprising an IGBT 13 to which a diode 14 is connected in a reverse parallel style, the upper and lower arms being series-connected. Series connection circuit 61 for a V-phase includes an upper arm comprising an IGBT 21 to which a diode 22 is connected in a reverse parallel style and a lower arm comprising an IGBT 23 to which a diode 24 is connected in a reverse parallel style. Similarly, a series connection circuit 62 for a W-phase includes an upper arm comprising an IGBT 31 to which a diode 32 is connected in a reverse parallel style and a lower arm comprising an IGBT 33 to which a diode 34 is connected in a reverse parallel style.
An AC switch is connected between the series-connection point of the upper arm and the lower arm of the series-connection circuit of each phase and the DC neutral point potential M, and an IGBT to which a diode is connected in a reverse parallel style is connected to the AC switch in a reverse series style. That is, an AC switch circuit is constructed so that an emitter of an IGBT module 63 (comprising an IGBT 81 to which a diode 82 is connected in a reverse parallel style) is connected to an emitter of an IGBT module 64 (comprising an IGBT 83 to which a diode 84 is connected in a reverse parallel style), and the AC switch circuit concerned is connected between the series-connection point of the series-connection circuit 60 for the U-phase and the neutral point M of the DC power source. Another AC switch circuit is constructed so that an emitter of an IGBT module 65 (comprising an IGBT 85 to which a diode 86 is connected in a reverse parallel style) is connected to an emitter of an IGBT module 66 (comprising an IGBT 87 to which a diode 88 is connected in a reverse parallel style), and the AC switch circuit concerned is connected between the series-connection point of the series-connection circuit 61 for the V-phase and the neutral point M of the DC power source. A further AC switch circuit is constructed so that an emitter of an IGBT module 67 (comprising an IGBT 89 to which a diode 90 is connected in a reverse parallel style) is connected to an emitter of an IGBT module 68 (comprising an IGBT 91 to which a diode 92 is connected in a reverse parallel style), and the AC switch circuit concerned is connected between the series-connection point of the series-connection circuit 62 for the W-phase and the neutral point M of the DC power source. The series-connection point of each of the series-connection circuits 60, 61, and 62 serves as an AC output, and the AC outputs are connected to a load 74 through filter reactors 71, 72, and 73.
With the circuit construction described above, the series-connection points of the respective series-connection circuits 60, 61, and 62 can output voltages at the positive terminal P, the negative terminal N, and the neutral point terminal M, and thus a 3-phase inverter output is obtained. FIG. 9 shows an example of an output voltage (Vout) waveform. This is characterized in that an AC voltage having three voltage levels and a little low-order harmonic component is output to another unit, and output filters 71 to 73 can be miniaturized.
Furthermore, when the 3-phase inverter is constructed as a current IGBT module, a 2-in-1 type IGBT module as shown in FIGS. 10A and 10B is used for the series-connection circuits 60, 61, and 62, and a 1-in-1 type IGBT module as shown in FIGS. 11A and 11B is used for the series-connection circuits 63 to 68. The 2-in-1 type module includes a C1 terminal (93) connected to the positive terminal P, an E2 terminal (94) connected to the negative terminal N, and an E1C2 terminal (95) connected to a load output and an AC switch, and the terminals are generally constructed in the order shown in FIG. 10A. As shown in FIG. 11B, the 1-in-1 type module has a collector terminal C(100) and an emitter terminal E(101) as output terminals.
FIG. 14 is a diagram showing the construction (top view) when one phase of the circuit in FIG. 8 using these IGBT modules. This circuit is constructed by the IGBT modules 60, 63, and 64 and with electrolytic capacitors 1 and 2 serving as DC power sources. These elements are connected to one another by electrical conductors. The circuit is the U-phase of the arrangement shown in FIG. 8. That is, the 2-in-1 type IGBT module 60 is used as the series-connection circuit, and the AC switch is implemented by an reverse series-connection circuit which is designed so that the emitters of the 1-in-1 type IGBT modules 63 and 64 are connected to each other. Electrical conductors 110 to 114 for connecting the DC power sources 1 and 2 and the IGBT modules 60, 63, and 64 are required. There are disadvantages in that the number of electrical conductors to be used is large, electrical conductors having complicated shapes are required, and also the inductance is increased. The form and the construction of the conventional IGBT modules are disclosed in “FUJI IGBT MODULE APPLICATION MANUAL,” RH984, issued in February, 2004.
FIG. 12 shows an equivalent circuit which is drawn by paying attention to the wire inductance of the circuit for one phase in FIG. 8. The respective inductances L1, L2, L3, L4 are mainly based on wiring between the IGBT modules and between the IGBT modules and the DC power source (electrolytic capacitors). Each electrical conductor normally ranges from about several centimeters to several centimeters in excess of ten, and thus each inductance value ranges from about 10 nH to about several tens of nH.
In FIG. 12, when IGBT 11 is turned off from an ON state (so that current I flows, indicated by a broken line), IGBT 81 (turned on in advance) and a diode 84 are conductive, and commutation is executed on a current passage Ix. At this time, a voltage in the direction indicated by an arrow in FIG. 12 transiently occurs in the inductances L1, L2, L3, L4 in accordance with the current variation rate (di/dt) of IGBT.
As a result, the voltage represented in the following expression (1) is applied between the collector and the emitter of IGBT 11 at maximum. FIG. 13 shows the waveforms of the collector current (ic) and the voltage (VCE) between the collector and the emitter when IGBT 11 is turned off.VCE(peak)=Edp+(L1+L2+L3+L4)·di/dt  (1)Surge voltage ΔV=(L1+L2+L3+L4)·di/dt  (2)
Edp: DC voltage of DC power source 1.
di/dt: current variation rate of the IGBT when it is turned off.
L1, L2, L3, L4: inductance values of respective conductors.
As an example, in the case of an IGBT that with a rating of several 100 A, di/dt is equal to about 5000 A/μs at maximum. Therefore, assuming that L1+L2+L3+L4=100 nH is satisfied, the voltage corresponding to the surge (L1+L2+L3+L4) di/dt in the expression (1) is equal to 500V.
As described above, due to existence of L1, L2, L3, L4, the peak voltage value applied to an IGBT when it is turned off is increased to be higher than the DC voltage Edp by the amount corresponding to the surge voltage of the expression (2). Therefore, the IGBT chip and an FWD chip connected in a reverse parallel style are required to have high withstand voltages (high voltage tolerance). Normally, the chip area of a chip having a high withstand voltage is generally increased in proportion to the withstand voltage. Therefore, there is a problem in that the module must be designed to be fairly large and the cost thereof is increased.